1. Field of the Invention
The present invention relates to a vehicle communication system including a ground device having multiple antenna units for setting specified communication areas on a road, and an on-vehicle device mounted on a vehicle for communicating with the ground device when the vehicle passes through the communication area.
2. Description of Related Art
An automatic toll charging method for a toll road such as an expressway has been provided where a vehicle has an on-vehicle device which stores a pre-registered identification number or the like and is capable of transmitting it, and a ground device is installed at a place for toll charging on the road so that communication is performed between the ground device and the on-vehicle device when the vehicle passes through a communication area set by the ground device, and the ground device stores an indication that the vehicle having the registered identification number has passed through the area, thereby performing toll charging by other means based on the recorded data.
For example, such a system, i.e., an unmanned system for a toll road, is disclosed in Japanese Laid-open Patent Publication No. Sho 55-116176. The system has entrances and exits on the toll road provided with transmitting and receiving systems, and a vehicle also has a transmitting and receiving system so that communication is performed therebetween to exchange tolling data and perform automatic toll charging without stopping the vehicle. Automatic tolling systems are also disclosed in Japanese Laid-Open Patent Publication Nos. Hei 5-314325, Hei 4-303289, Hei 4-315282 and Hei 6-131509.
The above systems are designed for a single traffic lane and therefore have some difficulties in application to a typical load having multiple lanes where a wide communication area is set. A toll charging system adapted to a road having multiple lanes is disclosed in, for example, Japanese Laid-Open Patent Publication No. Hei 5-324967. This toll charging system using a non-contact IC card is designed to improve the reliability of radio communication between the ground device and the non-contact IC card at a check barrier, where multiple antennas are connected to the ground device and are arranged so that the communication areas of the antennas are shifted from each other in the vehicle traveling direction with respect to a single lane, thereby increasing the occasions in which communication is possible between the ground device and the non-contact IC card of the vehicle.
In a toll road such as an expressway or the like where toll charging is needed, this construction eliminates the need for performing the toll charging with the individual lanes being partitioned in the toll road if the road has multiple lanes, but performs the toll charging automatically for the multiple lanes, thereby reducing the manpower and the incidence of traffic congestion at toll gates.
The above-described conventional construction requires that antennas be arranged over a long distance, so it is necessary that the antennas can be arranged even in a narrow installation space. Accordingly, the following construction can be employed, where the antennas can be arranged over a reduced distance using a similar method.
In example constructions described in U.S. Pat. No. 5,525,991 and U.S. patent application Ser. No. 08/574,635, multiple antenna units form communication areas corresponding to individual lanes of a road and the communication area of each antenna unit overlaps with the adjacent communication area, thereby enabling the communication areas of the antenna units to cover the entire width of the road without any communication blind spots. In this case, different signal transmission timings are set to the antenna units which have an overlapping area therebetween, and the signal frequencies of the antenna units are set to different values within a range in which on-vehicle devices can receive the signals, thereby enabling reliable communications.
In the above-described construction, however, frequency division is performed so that the frequencies of the antenna units are set differently. The construction therefore has a problem in that, taking a frequency band per antenna unit into consideration, the number of antenna units which can be arranged is limited if an available frequency band is limited.
Japanese Laid-Open Patent Publication No. Hei 6-243385 discloses a construction where the same frequency is set to respective antenna units and communication periods are set using a time-division multiplex method. This eliminates interference between the antenna units so that communication processing can be reliably performed although there are multiple antenna units using the same frequency.
In the above-described construction, however, when five antenna units 1 to 5 are used as shown in FIGS. 10A-10E, each communication period of the antenna units 1-5 is sequentially shifted by a period assigned by time division so that the antenna units 2-5 stop communication while the antenna unit 1 is communicating with an on-vehicle device 6 as shown in FIG. 11. Accordingly, one cycle time required for communications of all antenna units becomes 5T with respect to one-time communication time T of the antenna unit 1.
When the antenna units are set to the same frequency, as a result, if the antenna unit 1 is communicating with the on-vehicle device 6 which exists in a communication area 1a, as shown in FIG. 12, a communication signal output from an antenna unit 3 is also received by the antenna unit 1 where a communication area 3a is not overlapped, so that interference occurs while the antenna unit 1 is receiving a communication signal from an on-vehicle device 6 which exists in its communication area 1a, and this may lead to disable communication. The time-division multiplex communication is therefore performed to avoid such problems.
The communication time per antenna unit is inversely proportional to the number of antenna units, and the number of antenna units which can be used is also limited when considering a substantial time required for communication processing and the frequency of communication occurrence for reliably accomplishing communications with a vehicle which passes through the communication area, thus leading to a problem in that it is impossible to provide a number of antenna units for covering wide communication areas.